The cellular immune response against infections is initiated at the level of peptide loading and assembly of major histocompatibility complex (MHC) class I molecules. MHC class I presentation of nonself peptides triggers killing of infected cells by cytolytic T lymphocytes. The assembly of the MHC class I heavy chain with antigenic peptide and beta2-microglobulin (Beta2m) occurs via association with endoplasmic reticulum (ER) proteins such as calnexin, TAP, calreticulin, tapasin, and ERp57. Another cellular protein, amyloid precursor-like protein 2 (APLP-2), has recently been shown to associate with MHC class I molecules, and our preliminary findings indicate that APLP-2 down regulates the quantity of MHC class I molecules at the cell surface. The long-range goal of our laboratory is to comprehend the regulation of antigen presentation by MHC class I molecules. The objective of this Exploratory/Developmental Research (R21) Grant proposal is to define the effect of APLP-2 on the presentation of pathogen-derived epitopes. Our central hypothesis is that APLP-2 regulates MHC class I maturation and presentation of pathogen-derived peptides, including known epitopes from NIAID biodefense priority pathogens (Hantaan virus, Mycobacterium tuberculosis, influenza A, dengue virus, Japanese encephalitis virus, and Listeria monocytogenes). New insights obtained from this study will clarify the role of APLP-2 in the regulation of the MHC class I assembly pathway and may lead to new immune-based means to prevent or treat infections. The Specific Aims of this proposal are: Aim 1. To ascertain the cellular location of interacting APLP-2/MHC class I molecules and the influence of APLP-2 on MHC class I presentation of peptide. We hypothesize that APLP-2 regulates MHC class l peptide presentation at a late stage in MHC class I maturation. Aim 2. To determine the effect of APLP-2 on T lymphocyte recognition of pathogen epitopes. We hypothesize that the presentation of epitopes from NIAID priority pathogens is affected by APLP-2 interaction with the MHC class I molecule.